This invention relates to a copper-based sliding member for a bearing or the like, and more particularly to a copper-based sliding member excellent in resistance to corrosion from a sulfur-containing lubricant.
Recently, in some cases, an additive having a high sulfur content has been added to, for example, a lubricating oil such as an engine oil or the like for the purpose of diminishing the friction between the cylinders and the pistons. When such a lubricating oil having a high sulfur concentration is used, in the case of a copper-based bearing to be used in connecting-rod or the like, the Cu phase which is the matrix of the sliding layer of the bearing tends to be preferentially corroded by the sulfur, and there has been caused such a problem that the corrosion resistance of the sliding layer is not sufficient.
A copper-based bearing whose resistance to corrosion from sulfur has been increased is disclosed in JP-A 5(1993)-279,772. In this copper-based bearing, the composition of the sliding layer is such that the Zn content is 10 to 35% by weight, the Pb content is 2 to 20% by weight, the Ni content is 1 to 10% by weight, the B content is 0.1 to 1% by weight and the balance is composed of Cu.
Moreover, JP-A 8(1996)-42,572 discloses a bearing in which a sliding layer consisting of 28 to 32% by weight of Zn, 0.7 to 1.3% by weight of Si and the balance composed of Cu is bonded to a backing metal layer composed of steel.
The present inventors have made extensive research on the relation between the zinc content in the sliding layer and the amount of the sliding layer corroded with sulfur and have consequently found that the higher the zinc content was, the lower the amount of the sliding layer corroded with sulfur became; in other words, the resistance of the sliding layer to corrosion with sulfur is enhanced by an increase of the zinc content.
However, when it is intended to sinter or cast the sliding layer having a high zinc content onto the backing metal layer, the evaporation of zinc is violent and hence there have been such problems that it becomes difficult to bond the sliding layer onto the backing metal layer and the working environment becomes bad. Therefore, the zinc content has been limited up to 35% by weight, and hence, it has been impossible to prepare a copper alloy bearing having sufficiently high corrosion resistance.
Accordingly, JP-A 5(1993)-279,772 and JP-A 8(1996)-42,572 referred to above define the zinc content as not more than 35% by weight and not more than 32% by weight, respectively. Therefore, it has been impossible to prepare a copper-based sliding member having sufficient resistance to corrosion with sulfur.